Hard disk type magnetic memories have become one of the more commonly used systems for the storage of large quantities of data in computers. Up to 1.6 gigabits of data have been stored on a single disk about 6 inches in diameter. Hard disk memories usually include a rigid disk having a magnetic recording material applied to either or both of the flat surfaces of the disk. Small read/write heads are disposed adjacent these coated surfaces to access data on the disk. Typically, the read/write heads can either impress a magnetic field onto a segment of the disk as data is "written" onto the disk or sense the polarization of an existing magnetic field on the disk as data is "read" from the disk. A generally conventional hard disk type data storage system is illustrated in FIG. 1. The hard disk system 10 includes a plurality of rigid disks 12 attached at their centers to a spindle 15 with a series of read/write heads 18 flexibly coupled to a servo system 20 disposed adjacent the hard disks 12. Usually one read/write head 18 is associated with each flat surface 12a of the hard disks. In operation the disks 12 commonly rotate at speeds of several thousand revolutions per minute with the read/write heads 18 floating above the surfaces of the disks on a boundary layer of air about a millionth of an inch thick. Pivotal motion of the servo system 20 alters the position of the read/write heads 18 relative to the center of the disks 12, affording the read/write heads 18 coverage of the flat surfaces of the spinning disks 12.
The data recording surfaces of the hard disk are normally divided into numerous circular regions called tracks which are in turn subdivided into sectors. A sector normally takes up a predetermined portion of the arc of a single circular track. Typically, one surface on one of the hard disks 12 has a pre-recorded pattern dedicated to providing positional information for the heads 18. This dedicated hard disk surface is commonly referred to as the servo surface. Since the heads 18 move in unison, the positional information stored on the dedicated servo surface may be used to position any of the heads 18 with respect to any desired data track on their associated hard disk data surface. As data storage densities on hard disks continues to increase, however, the width of each track on the hard disk continues to decrease. Such tracks are now on the order of half a thousandth of an inch wide. The trend towards increasingly narrower track widths and increased data densities, however, has led to erroneous positioning of the read/write heads 18 when, for example, the dimensions of the hard disks 12 or the pivot arms for each of the read/write heads 18 changes even minutely due, for example, to thermal expansion. Errors in positioning the read/write head can then result, causing data errors and data recovery delay as the actual position of the track is sought.
Previous efforts have been attempted to correct for erroneous track location signals in hard disk control systems. U.S. Pat. No. 4,454,549 to Pennington, for example concerns a system for the calculation of track position displacement that relies on signals encoded on the disk to indicate the start of a sector. Two groups of "start of sector" signals are imprinted side by side along a circumferential arc oriented at an angle to the circumferential arc of the data portion of the sector. Measurement of a preponderance of either signal group is then employed to calculate a track position correction signal. This approach, however, by necessitating the encoding of start of sector signals onto each sector of a data track, disadvantageously reduces the amount of space within each track available for the storage of actual magnetic memory data. Imprinting start of sector signals on the hard disk also renders the Pennington approach susceptible to defects in the magnetic recording media of the hard disk. Media defects can either mask a true start of sector signal or, alternatively, masquerade as a false start of sector signal. Either condition may exacerbate problems in properly locating a desired sector or track and thus slow or otherwise hamper retrieval of data stored in memory. Thus, there still exists a need for an efficient track displacement sensing system that does not require encoding start of sector signals on hard disk data tracks but which can be implemented both efficiently and economically. The present invention fulfills this need.